The present invention relates to angular rate sensors for measuring small angular displacements. Specifically, a self-contained MHD sensor is provided which measures the angular rate of displacement about a central axis from one hertz to 2,000 Hz.
Angular motion sensors are known from U.S. Pat. No. 4,718,276 and U.S. Pat. No. 5,067,351. These angular motion sensors operate by measuring the electric potential produced from the relative motion between a liquid proof mass such as mercury and an outer case due to magneto-hydrodynamic effects produced from a magnetic field which is perpendicular to the rotational velocity of the sensor. Angular motion sensors are used in a variety of applications where stabilization of a platform is necessary. By employing three of these sensors along mutually perpendicular axes, it is possible to measure the angular displacement of the platform. In semiconductor microlithography processes, small vibrational displacement to a production surface can cause significant errors between a semiconductor mask and a substrate. The surface can be stabilized if effective measurement of the small displacements which accompany the vibration on the surface are possible. Using these measurements, a restoring force may be applied to the surface. Platform stabilization is also useful in eliminating the effects of angular motion in line of sight imaging systems. Typically, these line of sight imaging systems are comprised of precision gimbles, or fast steering mirrors and are stabilized using gyroscopes. The ability to compensate for the net displacement requires an ability to measure the small angular displacements which produce the instabilities in the imaging system.
Satellite imaging systems also suffer from vibrational forces which are imparted along the optical axis of the imaging system. Due to the resulting angular motion about the optical axis, a blurred image may be observed. Compensation of such image distortion may be effected by measuring the angular motion which occurs during the imaging process, and using a time dependent blurring function derived from the angular motion to correct for image distortion.
Each of these applications requires a sensor which is capable of measuring microradian displacements along one or more axes. Further, in these applications, specifically in satellite imaging processes, space is at a premium and the sensor must be small and not introduce a magnetic moment which would interfere with other equipment.